If the bore (diameter) is 4.05", the radius (half the diameter) is 2.025".

The area of the piston is pi*(2.025^2), or 12.88 square inches.

Multiply by the stroke (height) of 3.78", and you get 48.7 ci per cylinder.

Multiply again by the number of cylinders (8), and you get 389.6ci

So...

Boring 0.030 over increases the diameter of each cylinder to 4.08", and the radius to 2.04".

The area of the piston is now 13.07 square inches.

The stroke is still the same (3.78"), so the volume of each cylinder is 49.42 ci (an increase of 0.72 ci)

Multiply by 8 cylinders, for 395.4, an increase of 5.8ci.

Amazing what one can figure out with basic arithmetic isn't it. I liked your solution. since the starting point was not given, and I didn't know it, I was about to offer a X more than it was solution. It paid to pay attention in math class.